When an unattended hydraulically driven generator has a spring-closed butterfly valve coupled to a fluid-actuated valve operator, the unit will go to overspeed in a very few seconds when power is lost. Such overspeed is undesirable and noisy and could cause damage to components in the system associated with the generator. A need, therefore, exists for an improved control apparatus which provides for removal of the generator from a power line when emergency conditions exist, such as a sudden loss of line voltage or an overvoltage surge.